Electrochemical cells invariably comprise at their fundamental level a solid or liquid electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. Gas diffusion electrodes are employed in a range of electrochemical devices, in which a gaseous reactant and/or product has to be diffused into and/or out of one of the cell electrode structures. They are designed to optimise the contact between the reactant and the electrolyte to maximise the reaction rate. Catalysts are often incorporated into gas diffusion electrode structures to increase the rates of the desired electrode reactions.
Gas diffusion electrodes are employed in many different electrochemical devices, including metal-air batteries, electrochemical gas sensors, electrosynthesis of useful chemical compounds, and in particular, fuel cells.
A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The hydrogen or methanol are oxidised at the anode and oxygen is reduced at the cathode. Both electrodes are of the gas diffusion type. The electrolyte has to be in contact with both electrodes and may be acidic or alkaline, liquid or solid, in nature. In proton exchange membrane fuel cells (PEMFC), the electrolyte is a solid proton-conducting polymer membrane, commonly based on perfluorosulphonic acid materials, and the combined structure formed from the membrane and the two gas diffusion electrodes is known as the membrane electrode assembly (MEA). The anode gas diffusion electrode is designed to be porous and allow the reactant hydrogen or methanol to enter the electrode from the face of the electrode exposed to the reactant fuel supply, and diffuse through the thickness of the electrode to the reaction sites which contain catalysts, usually platinum metal based, to maximise the electrochemical oxidation of hydrogen or methanol. The anode is also designed to allow electrolyte to penetrate through the face of the electrode exposed to the electrolyte and to also contact the same reaction sites. With acidic electrolyte types the product of the anode reaction are protons and these can then be efficiently transported from the anode reaction sites through the electrolyte to the cathode gas diffusion electrode. The cathode is also designed to be porous and allow oxygen or air to enter the electrode and diffuse through to the reaction sites. Catalysts are again commonly incorporated to maximise the rate of the reaction at the cathode reaction sites which combines the protons with oxygen to produce water. Product water then has to diffuse out of the electrode structure. The structure of the cathode has to be designed such that it enables the efficient removal of the product water. If water builds up in the electrode, it becomes more difficult for the reactant oxygen to diffuse to the reaction sites, and thus the performance of the fuel cell decreases.
Conventionally, the gas diffusion electrodes of the PEMFC, and indeed other devices, comprise many components and are typically made up of one, two or even more layers of these components. Typically the gas diffusion electrode will comprise one or more catalyst containing layers, which are supported onto a more rigid porous substrate layer. The catalyst containing layers enhance the desired electrode reactions and comprise a catalyst, which may be formed from a high surface area catalytic metal, often one of the precious metals, particularly platinum, either unsupported, as a metal black (for example U.S. Pat. No. 4,927,514, EP 0357077), or in a very high surface area form in which it is dispersed and supported on a high surface area electrically conducting gas porous carbon black or graphite (for example U.S. Pat. No. 4,447,505). The catalyst component may also be a non precious metal, such as one of the transition metals. In fuel cells which employ alkaline electrolytes, the cathode gas diffusion electrode can comprise catalysts based on macrocyclic compounds of cobalt (U.S. Pat. No. 4,179,359, EP 0 512 713). The catalyst layers may also comprise the high surface area carbon black itself, with no additional metal catalysts, in for example EP 0 026 995 where the catalyst layer for an air depolarised cathode in a chlor-alkali cell comprises carbon black materials.
The catalyst layers also comprise other non-catalytic components in addition to the catalyst material usually polymeric materials which acts as binders to hold the electrode layer together and may also perform an additional function in controlling the hydrophobic/hydrophilic nature of the final structure. In the PEMFC in particular, the catalyst layers can also comprise other polymeric materials, such as proton conducting polymers, including forms of the proton conducting electrolyte itself, which are often mixed with the catalyst components or coated onto the catalyst containing layers, from solutions of the proton conducting polymer.
These catalyst layers are usually formed into suitable mixtures of the components and deposited onto a suitable porous substrate, for example conducting carbon materials such as semi graphitised papers, cloths or foams, or particularly in the case of alkaline electrolyte systems, metal meshes such as nickel or stainless steel, or in the case of sensors, various forms of porous PTFE sheet. In the acid electrolyte PEMFC the substrate is usually based on carbon paper or woven cloth materials (EP 0 026 995). These materials generally have a high bulk fibre density of greater than 0.4 g/cm.sup.3. The primary role of the substrate is to act as a physical support for the catalyst containing layers and to provide an electrically conducting structure in direct contact with the catalyst layer. Additionally it also enables a mechanically stable gas diffusion electrode to be produced.
A major problem with conventional gas diffusion electrodes based on the carbon fibre paper substrates is the lack of flexibility due to the rigid substrate that is typically used. The conventional electrodes are consequently easily damaged on handling which leads to high reject rates during the electrode and MEA fabrication process. This obviously has an impact on cost. With conventional gas diffusion electrodes based on woven cloth substrates a problem concerns the lack of good dimensional stability, as the cloth can easily be stretched in the directions of the major planar faces (x and y directions). This can make the manufacturing of electrodes and MEAs using these substrates very difficult and therefore costly.
Furthermore the complexity of the conventional gas diffusion electrode requires a number of separate components such as the substrate and the catalyst layers to be brought together which results in a lengthy manufacturing process requiring a number of steps. Again, this increases the cost per unit of these gas diffusion electrodes to higher than is currently acceptable to make applications in power generation devices, such as fuel cells, commercially viable.